This invention relates to a device provided with an image capturing element, and it relates, for example, to an electronic still camera which can exactly measure the distance of a subject.
With an improvement of the electronic technologies, there have been developed electronic still cameras, wherein images picked up are transformed into digital data to be stored, and they are already on the market. A user can display the image picked up in electronic still camera on his personal computer and can make prints by his printer, thus, the electronic still camera has become to have a wide range of application.
However, being provided with a range finding device employing infrared rays or ultrasonic waves, an ordinary silver halide type camera is able to measure the distance to a subject by the range finding device, and to move a image capturing lens to the focus position based on the distance.
Contrarily to this, in a common electronic still camera, separately from or additionally to this range finding device, there is provided a device which is able to obtain a focus position of the image capturing lens by an image capturing element (hereafter referred to as an image capturing focusing device).
While driving the image capturing lens for focusing, the image capturing focusing device is to search the best focus position from the resulted image signal. In this specification, let it be assumed that driving the image capturing lens is called “scanning”. As for the merit of the image capturing focusing device, there are given that the focusing is performed by the function employed originally in the electronic still camera, without using the range finding device such as one employed in the silver halide type camera, and that the focusing is performed more accurately by picking up the image practically, comparing to the above-mentioned range finding device which may have a case that the radiated infrared rays are not reflected effectively.
However, when the image capturing focusing device is used, the fact that the luminance of the subject is low is one reason for the low focusing accuracy. The smaller amount of light of the subject received by the image capturing element CCD makes a difference between the noise component existing in each pixel of a CCD and the image signal obtained by the photoelectric conversion to be less. It is difficult to find the focus position based on the image signal that is not clearly different from the noise component.
As described above, the image capturing focusing device practically picks up the image of the subject, while driving the image capturing lens for focusing, and searches the best focus point as a focus position based on the resulted image signal. Therefore, the searching of the position takes much time. Accordingly, there is conducted a method to obtain the focus position quickly by finding a subject distance with the range finding device of the silver halide type camera, and by moving the image capturing lens in the prescribed range whose center is corresponding to the subject distance.
However, it is not always desirable to focus the main subject at which a photographer aims. For example, in the case of the stroboscopic photographing, when the subject is beyond the range covered by the electronic flash light (for example 5 m), an amount of the reflected light of this subject is low, and its image becomes unclear, even if the photographer focused it. If another subjects existing at the different distance are out of focus, the picture will probably become blurred as a whole. A problem is how to photograph in such a case.
For the problem that the image capturing focusing device takes much time for detection of the focus position, if the image monitoring area smaller than the image plane for photographing is set at the center of the image plane for photographing where the main subject is estimated to exist, and if the focus position is obtained based on the image signal from CCD corresponding to this image monitoring area, the processing time of the image signal can be shortened to some degree. However, setting the image monitoring area has a risk to find the focus position erroneously, because of a failure to catch the subject image. For example, when the near subject is aimed through the optical finder of the electronic still camera, the subject scope imaged on the light receiving plane of the image capturing element can be greatly different from the subject scope observed through the optical finder because of the influence of the parallax, though there is not big difference for the distant subject. In this case, there is a fear that the lens may be driven for focusing for the subject which is different from one aimed through the optical finder.
It is possible to photograph a subject through enlargement or reduction when the zoom lens is provided on the electronic still camera, but if the image monitoring area is set to be constant regardless of image magnification, when a subject is too small for the image monitoring area, the background for the subject may be focused, meanwhile, when the subject is too large for the image monitoring area, only the person's clothes of uniform graduation may be in the image monitoring area, and it is sometimes impossible to obtain the clear evaluation value for judging whether the subject is in focus or not.
Whereas, in an active range finding device, using the infrared rays or ultrasonic waves (hereafter referred to as infrared rays or others), the subject distance is measured by radiating the infrared rays or others, and by receiving the infrared rays or others reflected from the subject. When the subject is distant, the reflected infrared rays or others decrease, and when the infrared rays or others exist in abundance in the environment, the difference from the infrared rays or others reflected from a subject becomes unclear, so the measured distance is feared to be inaccurate.
Accordingly, if the scanning range of the image capturing lens is decided by the incorrectly measured distance, the resulted focus position may be inappropriate.
Additionally, when the exposure is excessive because of the characteristics of CCD, photoelectric converting pixels are saturated and the saturated pixels output nothing but fixed pixel signals, which makes it difficult to form the image based on these pixel signals. Accordingly, overexposure needs to be prevented in an electronic still camera. Though it is comparatively easy to prevent the overexposure for photographing under the natural light, it is difficult, in photographing employing electronic flash light, to estimate the graduation of the subject before photographing, so the problem is how to prevent the overexposure.